This invention relates to an objective lens driving device adapted to be used in an optical reproducing apparatus wherein digital information recorded on a disc is reproduced accurately, and more particularly to a device in which a damper mechanism can be inserted easily and the neutral position of the objective lens can be maintained precisely.
Recently digital recording and reproducing apparatus ratus utilizing PCM (Pulse Code Modulation) techniques have become popular in the fields of audio apparatus, because the audio input and output properties of the PCM digital apparatus are not adversely affected by the property of the recording medium and because the recorded and reproduced results thereof are absolutely free from noises. In a case where the recording medium is formed into a circular disc, reproducing apparatus of, for instance, the optical type, electrostatic type and mechanical type have been widely used. Regardless of these types, it is apparent that the reproducing apparatus utilizing the PCM technique requires operational characteristics of far more precision than those in the conventional type reproducing apparatus.
For instance, in the case of an optical type reproducing apparatus to be operated with a compact disc (CD), it is required to read out information recorded on the disc at a pitch of 1.6 microns precisely, and therefore the reading portion of the apparatus should be operable with high precision and dexterity.
In the optical compact disc reproducing apparatus, the information recorded on the disc is ordinarily read through an objective lens. For assuring reproduction of the information at high fidelity, the objective lens must be focused exactly to a groove or pit in which the information is recorded, and the position of the objective lens must be controlled precisely so that it follows a track of the disc without fail. Ordinarily, the focusing and tracking operations of the lens have been realized by holding the objective lens on a lens driving device, and by controlling the lens driving device by means of a servo system with reference to the information signal thus obtained.
The lens driving device which holds the objective lens, and carries out the focusing and tracking operations as described above has been constructed as shown in FIGS. 1, 2 and 3.
In the shown device, a stationary shaft 2 is secured to a central portion of a base plate 1 made of a magnetic material so that the shaft 2 extends vertically upwardly from the base plate 1. A cylindrical member 4 with an upper end thereof closed by a wall 4a is supported by the shaft 2 through a bearing member 3 of a sleeve-like configuration, which is secured vertically to a central portion of the end wall 4a so that the cylindrical member 4 is slidable along and rotatable around the stationary shaft 2.
The end wall 4a of the cylindrical member 4 supports an objective lens 5, while the circumferential wall 4b of the cylindrical member 4 is used as a coil winding bobbin. That is, a focusing coil 6 is wound around the circumferential wall 4b for controlling the vertical position of the cylindrical member 4 along the stationary shaft 2, while a suitable number of pairs of tracking coils 7 are provided at positions spaced apart along the circumferential wall 4b of the cylindrical member 4 for controlling the angular position of the cylindrical member 4 around the shaft 2. Furthermore, at positions symmetric with respect to the shaft 2, a pair of internal yokes 8a and 8b are provided so as to project upwardly from the base plate 1 passing the interior of the circumferential wall 4b in a non-contacting relation, until the upper ends of the internal yokes 8a and 8b are in opposition to the internal surface of the end wall 4a of the cylindrical member 4. A pair of external yokes 9a and 9b are provided on the other hand outwardly of the circumferential wall 4b in a relation opposing to the internal yokes 8a and 8b. Permanent magnets 10 magnetized in the axial direction (vertically as viewed in FIGS. 2 and 3) are interposed between the external yokes 9a and 9b and the base plate 1.
A vertical shaft 11 of a small diameter is secured at the lower end thereof to the base plate 1 so that the upper end of the shaft 11 opposes to the internal surface of the end wall 4a. A damping member 12 made of a flexible material such as rubber is extended between the shaft 11 with a small diameter and the sleeve-like bearing member 3 secured to the end wall 4a of the cylindrical member 4 for defining a neutral position of the focusing and tracking operation of the lens driving device. A light introducing hole 13 is further provided through the base plate 1 for transmitting light entering or coming out of the objective lens through the hole 13.
During the operation of the objective lens driving device, an electromagnetic force produced by energizing the focusing coil 6 moves the cylindrical member 4 vertically in the direction of Y axis for controlling the focus of the objective lens, while an electromagnetic force produced by the energization of the tracking coils 7 rotates the cylindrical member 4 around the central shaft 2 in the direction of, the X axis shown in FIG. 1 for realizing a tracking control of the objective lens. The energizations of the focusing coil 6 and the tracking coils 7 are carried out by a servo system, not shown.
In the above described conventional objective lens driving device, the damping member 12 which defines the neutral position during the focusing and tracking operations is made of a soft material such as rubber formed into an extremely small size. Accordingly, it is extremely difficult to secure the soft and small-size damping member 12 to the shaft 11 and the bearing member 3 so that the objective lens locates at the neutral position precisely, and the elimination of such difficulty has been urgently required